Nano-structure and method of making the same

ABSTRACT

In an example of a method for making a nano-structure, an aluminum layer is partially anodized to form a porous anodic alumina structure. The aluminum layer is positioned on an oxidizable material layer. The porous anodic alumina structure is exposed to partial anisotropic etching to form tracks within the porous anodic alumina structure. A remaining portion of the aluminum layer is further anodized to form paths where the tracks are formed. The oxidizable material layer is anodized to from an oxide, where the oxide grows through the paths formed within the porous anodic alumina structure to form a set of super nano-pillars.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. application Ser. No.13/822,070, filed Mar. 11, 2013, which is itself a 35 U.S.C. § 371national phase of International Application Ser. No. PCT/US2010/053578,filed Oct. 21, 2010, each of which is incorporated by reference hereinin its entirety.

BACKGROUND

The present disclosure relates generally to nano-structures and methodsof making nano-structures.

Porous anodic oxide structures may be used in a variety of applicationsincluding, for example, micro- and nano-electronics (such as, e.g., inplanarized aluminum interconnections, precision thin-film resistors,thin-film capacitors, and nano-structured field-emission cathodes),electrostatic and thermo-activated switching devices, LC high-frequencyoscillators, AC amplifiers, triggers and other logic vacuum integratedcircuits (VICs), gas micro- and nano-sensors, micro- and nano-channelplates, mesoscopic engines, wavelength-sensitive filters, reflective andabsorbing surfaces, membranes, nozzles, precision apertures, and/orlike. These anodic oxide structures may also include one or more arraysof nano-pores that are used, for example, to form one or more arrays ofnano-pillars formed on and supported by a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the present disclosure willbecome apparent by reference to the following detailed description anddrawings, in which like reference numerals correspond to similar, thoughperhaps not identical, components. For the sake of brevity, referencenumerals or features having a previously described function may or maynot be described in connection with other drawings in which they appear.

FIGS. 1A through 1F together schematically depict an embodiment of amethod of forming an embodiment of a nano-structure;

FIGS. 1A through 1E, 1G and 1H together schematically depict anotherembodiment of a method of forming another embodiment of anano-structure;

FIGS. 1C-A, 1D-A, 1E-A, and 1G-A are enlarged views of a portion of theschematic depiction shown in FIGS. 1C, 1D, 1E, and 1G, respectively;

FIG. 2 is a graph showing a voltage and current dependency onanodization time for aluminum anodization;

FIG. 3A is a schematic three-dimensional representation of the porousanodic alumina template of FIG. 1B;

FIG. 3B is a top view of the example of the anodic alumina template ofFIG. 3A;

FIG. 3C is a scanning electron micrograph (SEM) image showing across-section of an anodic alumina template after about 30 minutes ofetching;

FIG. 4 is a schematic depiction of an embodiment of a template havingsecondary pores formed in pores;

FIG. 5A is a transmission electron micrograph (TEM) image of across-section of the nano-structure formed using the embodiment of themethod shown in FIGS. 1A through 1F;

FIG. 5B is a SEM image of a perspective view of the nano-structure shownin FIG. 5A;

FIG. 6A is a TEM image of a cross section of the nano-structure formedusing the embodiment of the method shown in FIGS. 1A through 1E, 1G and1H; and

FIG. 6B is a SEM image of a perspective view of the nano-structure shownin FIG. 6A.

DETAILED DESCRIPTION

Embodiment(s) of the nano-structure disclosed herein includes at leasttwo sets of super nano-pillars. As used herein, the term “supernano-pillar” refers to a nano-pillar that is a fraction of the size (interms of effective diameter) of a single larger nano-pillar, while a“set of super nano-pillars” refers to a discrete cluster (i.e.,physically separated from another cluster) of super nano-pillars. As anillustrative example, one set is similar to a single larger nano-pillarthat is split into a plurality of smaller super nano-pillars. In anexample, each set includes from about 5 to about 10 nano-pillars. Inanother example, each set includes more than 10 nano-pillars (see, e.g.,FIG. 5B). As such, the size of each super nano-pillar ranges from about⅕ to about 1/10 of the size of the entire set (or of a single largernano-pillar).

The sets of super nano-pillars may advantageously impart a shockabsorbing property to the nano-structure such that the nano-structuremay be used as a shock absorber, a substrate for a micro- andnano-sensor, active media for a micro- and nano-reactor (such as alab-on-a-chip device), and/or the like. In many cases, devices made withor incorporating the nano-structure may be considered to be robust, atleast in part because of the intrinsic flexibility of thenano-dimensional structures disclosed herein. This is due, at least inpart, to the fact that the weakest part of the structure, i.e., thesuper nano-pillars, are built from amorphous oxide (i.e., nomicrocrystals with grain boundaries) and are built from the samematerial as the underlying support (i.e., no interfaces are presentbetween the super nano-pillars and the underlying dense oxide). As such,when exposed to external forces (e.g., mechanical pressure) the supernano-pillars bend rather than break.

Further, the presence of the super nano-pillars in discrete sets in thenano-structure also advantageously increases the active surface area ofthe structure, as well as its mechanical flexibility. In some instances,the sets of super nano-pillars also impart at least some additionalfunctionality to the nano-structure itself. For example, due, at leastin part, to the significant curvature of their surface (i.e., diameteris very small), it is expected that enhanced surface energy andcatalytic performance should be exhibited by the super nano-pillarsdisclosed herein.

Embodiments of the nano-structure disclosed herein may also be used as asuperhydrophilic or superhydrophobic surface for a micro- andnano-fluidic device. As used herein, a “microfluidic device” refers to adevice for capturing or separating micrometer-sized or smallerparticulates within a fluid sample, whereas a “nanofluidic device”refers to a device for capturing or separating nanometer-sized orsmaller particulates within a fluid sample. Examples of micro- ornano-fluidic devices include lab-on-a-chip devices, devices for thedetection of an analyte, and devices for separating or sensing. In oneembodiment, the device shown in FIG. 1H may be suitable for use as aseparating or sensing unit.

Referring now to the figures, one embodiment of the nano-structure 100is schematically depicted in FIG. 1F, and the formation of thisnano-structure 100 will be described below in conjunction with FIGS. 1Athrough 1F. Another embodiment of the nano-structure 100′ is depicted inFIG. 1H, and the formation of this nano-structure 100′ will be describedbelow in conjunction with FIGS. 1A through 1E, 1G and 1H. While multiplesets 24, 24′ of nano-pillars are shown in FIGS. 1F and 1H, it is to beunderstood that the method may be revised in order to form one set 24,24′ of nano-pillars or any desirable number of sets 24, 24′ ofnanopillars.

Further, as shown in FIGS. 1F and 1H, the nano-pillars 20 in each set24, 24′ of the nano-structure 100, 100′ is positioned on an oxidizedlayer 14′, which is formed from an oxidizable material layer 14deposited on a substrate 12.

An embodiment of the method of forming the nano-structure 100 will nowbe described herein. Referring now to FIG. 1A, the method of forming thenano-structure 100 includes forming a multi-layered structure 10 thatcontains i) an oxidizable material layer 14 established on a substrate12, and ii) another oxidizable material layer 16 established on theoxidizable material layer 14. The multi-layered structure 10 may beformed, for example, by depositing an oxidizable material on thesubstrate 12 to form the oxidizable material layer 14 having athickness, and then depositing the other oxidizable material on theoxidizable material layer 14 to form the other oxidizable material layer16, which also has a thickness. In one embodiment, the oxidizablematerial layer 14 is formed of a metal or metal alloy that forms a denseoxide after electrochemical oxidation, and the other oxidizable materiallayer 16 is formed of a metal or metal alloy that forms a porous oxideafter electrochemical oxidation. Examples of suitable materials will bediscussed further hereinbelow.

The layers 14, 16 have respective thicknesses that may be different ormay be substantially the same. In one embodiment, the thickness of eachof the layers 14, 16 is in the nanometer range. The layer 14 may haveany suitable thickness that will produce (during electrochemicaloxidation) enough oxide to form the nano-pillars 20 and, in someinstances, the cap layer 22 formed over the nano-pillars 20 (which willbe described in further detail below, see FIGS. 1G and 1H). In anexample, the thickness of the oxide grown from the layer 14 (i.e., thestructure 14′, the nano-pillars 20, and, in some instances, the caplayer 22) is determined by multiplying the anodization voltage by ananodization coefficient (i.e., the number of nanometers that the oxidegrows per one volt of anodization voltage). For instance, for a Ta layer14, about 1.8 nm of Ta₂O₅ grows per volt of anodization voltage appliedto the layer 14 to form a dense Ta₂O₅ film. In another instance, fromabout 1.3 nm to about 1.8 nm of Ta₂O₅ grows per volt of anodizationvoltage applied to the layer 14 to form nano-pillars with an underlyingdense Ta₂O₅ film and with a dense Ta₂O₅ cap layer. In the latterinstance (i.e., when the cap layer 22 is formed and there is a denseTa₂O₅ film 14′ beneath the nano-pillars 20), the anodization coefficientdepends, at least, on the diameter of the paths (discussed below) in thetemplate 16′, the overall porosity of the template 16′, the nature ofthe electrolyte used for Ta anodization, and the current density duringTa anodization.

The thickness of the Ta layer 14 should be thick enough to produce aTa₂O₅ layer having any desired thickness and, in some cases, to maintainsome of the Ta layer 14 on the substrate 12 underneath the formed supernano-pillars 20 and the dense portion of Ta₂O₅. For example, to producea dense Ta₂O₅ layer with no nano-pillars, the total thickness of the Talayer 14 may be calculated by i) multiplying the completed anodizationvoltage (i.e., the specific voltage at which the oxide thickness reachesa steady state value) by 1.8 nm of Ta₂O₅ growth per volt to determinethe thickness of Ta₂O₅ that grows, and ii) then dividing that number bythe expansion coefficient (i.e., the ratio of Ta₂O₅ to consumed Ta),which is 2.3. For instance, if an anodization voltage of 200 V is usedfor completing the Ta anodization and 1.8 nm of Ta₂O₅ grows per volt(which produces about 360 nm of Ta₂O₅), and the expansion coefficient is2.3, then the thickness of the Ta layer 14 is about 160 nm. In instanceswhere nano-pillars, with an underlying dense Ta₂O₅ film, are grown fromthe Ta layer 14 (with or without a cap layer), the thickness of the Talayer 14 is based, at least in part, on the volume of Ta₂O₅ (whichdepends, at least in part, on the fraction of pillars in the entirestack, as well as their filling factor) and the anodization coefficient(which depends, at least in part, on the electrolyte used and theanodization conditions, and is from about 1.3 nm to about 1.8 nm pervolt for tantalum).

The thickness of the layer 16, on the other hand, should be thick enoughto form a template 16′ (see FIG. 1B) that has a height that is greaterthan the height of the super nano-pillars 20 to be grown from the layer14. In one example, the layer 16 has a thickness of 100 nm or less. Inanother example, the layer 16 has a thickness of 50 nm or less. In anexample, the thickness of the template 16′ is about the thickness of thelayer 16 times the expansion coefficient (e.g., about 1.3, which is theratio between the thickness of the porous anodic alumina and thethickness of the aluminum layer 16 consumed).

In an example, each of the layers 14, 16 are planar (e.g., aresubstantially flat and include, if any, a minimal amount ofirregularities). In another example, one or more of the layers 14, 16are non-planar. In this example, the non-planar layer(s) 14, 16 may alsoinclude a special morphology, features, structures, and/or the like thatare etched into or other incorporated into the layers 14, 16. The planaror non-planar layers 14, 16 may be deposited on a planar or non-planarsubstrate 12, which will be described further below.

The deposition of the oxidizable material on the substrate 12 and thedeposition of the other oxidizable material on the oxidizable materiallayer 14 may be accomplished using any suitable deposition techniqueknown in the art. Some examples of suitable deposition techniquesinclude physical vapor deposition (PVD) (such as, e.g., sputtering,thermal evaporation, and/or pulsed laser deposition), atomic layerdeposition (ALD), or, in some instances, chemical vapor deposition(CVD).

The substrate 12 upon which the oxidizable material is deposited to formthe layer 14 may be chosen based, at least in part, on the applicationfor which the nano-structure 100 will ultimately be used. If, forexample, the nano-structure 100 is to be used for semiconductorapplications, the substrate 12 may be chosen from suitable supportstructures for semiconductors such as, e.g., a substantially planarsilicon wafer. By “substantially planar”, it is meant that the surfaceis flat but may contain some irregularities. In this example, thesubstrate 12 may have formed thereon a layer of insulating material (notshown) such as, e.g., silicon oxide or silicon nitride. The substrate 12may also or otherwise be a non-planar structure, e.g., the substrate 12may have a special morphology etched on or fabricated into the substrate12. The substrate 12 may also be chosen from other materials such as,e.g., glass, quartz, alumina, stainless steel, plastic, and/or the like,and/or combinations thereof. In instances where the nano-structure 100is used as a nano-filter, the substrate 12 may be chosen from a Si waferwith a thermally grown oxide (TOX) layer thereon, such as TOX/Si orSiO₂/Si. In an example, TOX/Si may be formed by oxidizing Si at a hightemperature (i.e., from about 800° C. to about 1200° C.) using watervapor (steam) or molecular oxygen as the oxidant. In other words, TOX/Simay be formed via dry or wet oxidation, and the TOX/Si oxide layer maybe referred to as a high temperature oxide layer. In some cases, a dryoxygen atmosphere produces a higher quality SiO₂, but the process itselfis relatively slow. For thicker TOX/Si layers (i.e., a thickness ofabout 0.5 μm to about 4 μm or more), oxidation of the Si in a wet oxygenatmosphere is desirable. Other examples of suitable substrates include,but are not limited to, SiN, SiC, TEOS (which is SiO₂, but is preparedusing a chemical vapor deposition (CVD) method fromtetra-ethyloxy-silane (i.e., tetra-ethyl-ortho-silicate)), or the like.

The oxidizable material for the oxidizable material layer 14 is aconductor and may be chosen from a material that i) can beelectrochemically oxidized and ii) has an expansion coefficient, duringoxidation, that is more than 1. In some cases, the oxidizable materialfor the layer 14 may also or otherwise be thermally oxidized. Withoutbeing bound to any theory, it is believed that an expansion coefficientof more than 1 allows the oxidizable material to squeeze into the paths32 of the template 16′ (which will be described in further detailbelow). It is further believed that the height of the super nano-pillars20 that are formed may, at least partially, be based on the expansioncoefficient of the material in the layer 14. In an example, a supernano-pillar 20 height ranging from about 10 nm to 500 nm may be achievedwhen the expansion coefficient of the oxidizable material in layer 14 ismore than 1. It is to be understood that the height of each of the supernano-pillars 20 (including the thickness of structure 14′) may also bebased, at least in part, on other factors including the anodizationvoltages used during the respective anodization of layers 16 and 14.Some examples of suitable oxidizable materials include tantalum (whichhas an expansion coefficient for thermal oxidation of 2.3, as mentionedabove), titanium (which has an expansion coefficient for thermaloxidation of 1.7), niobium (which has an expansion coefficient forthermal oxidation of 2.7), and tungsten (which has an expansioncoefficient for thermal oxidation of 3.3). It is to be understood thatthe expansion coefficient for thermal oxidation for each of theforegoing materials is substantially the same as that forelectrochemical oxidation so long as the phase of each of thesematerials during oxidation is the same.

The other oxidizable material for the other oxidizable material layer 16is also a conductor, but is chosen from a metal or metal alloy that,after electrochemical oxidation, produces a porous oxide. One example ofthe other oxidizable material includes aluminum or aluminum alloys, suchas an aluminum alloy having aluminum as the main component. It isfurther to be understood that silicon, titanium, tantalum, niobium, andtungsten in the aluminum alloy may be present in small quantities suchas, e.g., up to about 5%. It is believed that any porous material couldbe used for similar super nano-pillar fabrication if the etching of suchmaterial is accompanied by the creation of etching tracks 31 (seediscussion of FIG. 1C).

The oxidizable material forming the oxidizable material layer 14 and theother oxidizable material forming the other oxidizable material layer 16are substantially pure. As used herein, the term “substantially pure”refers to a material (such as a metal or a metal alloy) having a minimalamount, if any, impurities present therein. In an example, asubstantially pure metal may be one that includes at least 95% of themetal. In some cases the substantially pure metal includes about 100%metal, and thus practically no impurities. In these cases, the metal maybe referred to as a substantially pure metal, a pure metal, or just ametal. In an example, the substantially pure metal has at least about a99.9% (e.g., often expressed as 3N), and in some cases at least about99.99% purity (e.g., often expressed as 4N). It is to be understoodthat, in some instances, the oxidizable material and/or the otheroxidizable material may be a metal alloy.

For purposes of illustration, the method depicted in the FIG. 1 serieswill be described using tantalum as the oxidizable material in theoxidizable material layer 14, and aluminum as the oxidizable material inthe other oxidizable material layer 16. As such, in reference to FIGS.1A through 1H, the layer 14 will be referred to as the tantalum layer14, and layer 16 will be referred to as the aluminum layer 16. However,as previously noted, the layers 14 and 16 are not to be construed asbeing limited to being formed of tantalum and aluminum, respectively,but can be any of the oxidizable materials listed herein for therespective layers 14, 16.

After the multi-layered structure 10 is formed, a template 16′ is formedby partial anodization of the aluminum layer 16 (as shown in FIG. 1B).In the embodiments disclosed herein, the template 16′ is modified byother processes, including partial etching and additional anodization.As used herein, partial anodization refers to the oxidation of a part ofthe thickness of the layer being anodized. In an embodiment, thethickness of the aluminum layer 16 that is partially anodized toinitially form the template 16′ (as shown in FIG. 1B) is determined bydetermining an amount of anodization time needed to completely anodize(i.e., to fully oxidize the layer), and then performing anodization forsome time less than the amount of time needed to completely anodize. Assuch, the amount of time to partially anodize the layer 16 is estimatedfrom the amount of time that will completely anodize the layer 16. In anexample, the amount of anodization time needed to completely anodize thealuminum layer 16 (which may be referred to herein as “time zero”) isestimated by analyzing a dependency of anodization voltage and currentdensity on an anodization time of the aluminum layer 16, as shown in thegraph in FIG. 2. In the example shown in FIG. 2, the layer 16 was a 300nm thick aluminum layer and the layer 14 was a 50 nm thick tantalumlayer. The anodization area was 3 cm², the electrolyte was 4% wt. %oxalic acid, and the cathode was 4N Al. Anodization was performed inpotentiostatic regime, with the voltage being 15V. It is to beunderstood that if different conditions are utilized (e.g., Althickness, electrolyte, etc.), the numbers will be different. As shownin FIG. 2, in this example, the current density begins to drop at about480 seconds of anodization time of the layer 16, and continues to dropuntil about 550 seconds of anodization time. At about 550 seconds, thecurrent drops to a steady state value, and anodization of the layer 16is considered to be complete. Several options may be used to estimatetime zero from the graph of FIG. 2. For instance, time zero may be whenthe aluminum layer 16 is completely anodized (e.g., at 15% of the steadystate anodization current, which occurs at about 550 seconds, asidentified by option 1 in the graph), or at various times prior butclose to complete anodization of the aluminum layer 16, which are easyto estimate based on the experimental data (e.g., at 480 seconds, 510seconds, or 540 seconds, which are options 2, 3, and 4 shown in thegraph, respectively).

It may be difficult to identify the exact time at which the currentbegins to decrease, and thus the time may be estimated at plus or minus5 to 10 seconds. The options shown in FIG. 2 are the result ofextrapolations. The best fit line L1 is associated with the regionbetween about 100 seconds and about 450 seconds, the best fit line L2 isassociated with the region between about 550 seconds and about 600seconds, and the best fit line L3 is associated with the region betweenabout 490 seconds and about 540 seconds. The options at theintersections between the best fit lines L1 and L3 (i.e., option 2) andL2 and L3 (option 3) are more precise in terms of process duration thanoption 1. Option 4 represents half the distance between options 2 and 3.When the minimal current that can be obtained for the system is knownand anodization current can be observed throughout the process, thenoption 4 can be readily calculated. For example, if anodization istaking place at 5 mA and the final current is 0.5 mA, option 4 will bewhen the current drops down to 2.75 mA [(5−0.5)/2+0.5]. In order to stopin advance of this calculated complete anodization, it may be desirableto run test experiments, estimate the durations of each region, andcalculate a desired stop time.

The template 16′, shown in FIG. 1B, is formed by partially anodizing thealuminum layer 16 using an amount of anodization time determined fromthe graph in FIG. 2. In an example, partial anodization of the layer 16is accomplished for less than 480 seconds. It is to be understood thatenough of the aluminum layer 16 is anodized to form the desired template16′, which includes a plurality of pores 18 defined therein and abarrier layer B of alumina that defines the bottom of each pore 18. Asillustrated in FIG. 1B, after template 16′ formation, there is someremaining non-anodized aluminum 16. Partial anodization of the aluminumlayer 16 to form the template 16′ may be accomplished by employing thealuminum layer 16 as the anode of an electrolytic cell and selecting atleast one of H₂SO₄ (sulfuric acid), H₃PO₄ (phosphoric acid), C₂H₂O₄(oxalic acid), or H₂CrO₄ (chromic acid) as the electrolyte. Theelectrolyte may be present in a water based solution. These electrolytesform porous alumina rather than dense alumina. In one embodiment wherethe electrolyte is oxalic acid (C₂H₂O₄), the electrolyte may be present,in solution with water, at a wt % ranging from about 1 wt % to about 5wt %. In another embodiment where the electrolyte is sulfuric acid(H₂SO₄), the electrolyte may be present, in solution with water, at avol % ranging from about 5 vol % to about 40 vol %. In some instances,certain additives (e.g., an alcohol, a surfactant, etc.) may also beadded to the electrolyte solution. It is to be understood that theconcentration of electrolyte in solution and the other conditions mayvary as long as they are suitable for porous anodization (i.e., theformation of the porous template 16′). Any suitable cathode may be used,for example, aluminum or platinum (e.g., foil or mesh). A suitableamount of voltage and current is then applied to the electrolytic cellfor an amount of time to partially anodize the aluminum layer 16 (i.e.,where the anodized portion of the aluminum layer 16 is oxidized). Theanodization of the aluminum layer 16 forms porous anodic aluminum oxide(i.e., porous anodic alumina), and allows the alumina to grow to adesired thickness.

The porous template 16′ is shown in FIG. 1B (cross-sectional view), 3A(perspective view), and 3B (top view), and 3C (a SEM image of a crosssection of the template 16′). It is to be understood that the pores 18at this point in the process do not extend through to and expose theunderlying tantalum layer 14. The template 16′ includes a plurality ofcells 17 (see FIGS. 3A and 3B) each having a pore 18 defined therein. Inan example, each of the pores 18 defined in the template 16′ is orientedsubstantially normal to the substrate 12 surface. The size of the pores18 formed during anodization may be controlled through the selection ofthe electrolyte and the anodization conditions. For instance, for analumina template, the diameter D of a cell 17 is about 2.8 nm per volt(e.g., when Al is used for layer 16), and the diameter d of the pore 18depends on the electrolyte and the current density. In one embodiment,the diameter d of the pore 18 is proportional to the voltage used. Theratio of the cell diameter and the pore diameter (D/d) is, for example,3.3 for a H₂CrO₄ electrolyte, 4.9 for a H₂SO₄ electrolyte, 3.0 for aH₂C₂O₄ electrolyte, and between 1.7 and 2.1 for a H₃PO₄ electrolyte. Asexamples, pores of the following sizes may be obtained using thefollowing electrolytes: pores having about 20 nm diameters may beobtained using H₂SO₄ as the electrolyte, pores having about 40 nmdiameters may be obtained using C₂H₂O₄ as the electrolyte, and poreshaving about 120 nm may be obtained using H₃PO₄ as the electrolyte.

In another embodiment, prior to performing anodization, the methodincludes patterning the aluminum layer 16. Patterning may beaccomplished via any suitable technique, and is used to performlocalized anodization of the aluminum layer 16. Any standardphotolithography method may be utilized. One example of patterning withstandard photolithography includes depositing a hard mask material(e.g., Si_(x)N_(y) such as SiN or Si₃N₄) on the aluminum layer 16, andthen using a photoresist to pattern the Si_(x)N_(y) material to allowlocalized exposure of aluminum. In an example, the mask is patterned toexpose portion(s) of the aluminum to the electrolyte. In some cases, thealuminum may also be patterned and etched to produce clusters ofaluminum (i.e., formed when areas of aluminum are etched, but the Ta isstill present). In other cases, aluminum and tantalum are patterned andetched to produce clusters of aluminum/tantalum. In this example, theinterface formed between the mask and the aluminum layer 16 is robust,which advantageously prevents separation of the layers duringanodization. In one embodiment, the areas that remain exposed once themask and photoresist are in position are subject to local anodization.The aluminum layer exposed via the patterned mask or the patternedaluminum layer (not shown) is then locally anodized, for example, byemploying the exposed or patterned aluminum layer as the anode of anelectrolytic cell, and employing any suitable cathode, such as aluminum(having a 99.99% purity) and/or platinum (foil or mesh). The electrolytemay be selected from any electrolyte that will suitably allow theformation of porous alumina. Some examples of the electrolyte includesolutions of H₂SO₄, H₃PO₄, H₂C₂O₄, and/or H₂CrO₄. A suitable voltage andcurrent is then applied to the electrolytic cell for an amount of timeto partially anodize the patterned aluminum layer. The combination ofpatterning and anodization forms a porous anodic alumina template 16′with specific dimensions.

In one example, the anodization of the aluminum layer 16 may beaccomplished via a potentiostatic regime, whereby a constant anodizationvoltage is applied. Due at least in part to the pore diameter beingproportional to voltage, anodization using a constant voltage producespores having a substantially constant diameter from top to bottom. Inanother example, the anodization may be accomplished via a galvanostaticregime, whereby a constant current density is applied, and thus aconstant rate of anodization is achieved. In one example, the voltagemay vary during the anodization, which produces pores 18 having avarying diameter from top to bottom. Varying the voltage during template16′ formation may also lead to the formation of secondary pores 18′,18″. The formation of secondary pores 18′, 18″ is shown in FIG. 4. Theinitial anodization voltage may be decreased to create one or more newsubstantially parallel pores 18′, 18″ per every initial pore 18. Thenumber of secondary pores 18′, 18″ may be readily controlled via theanodization voltage. All of these secondary pores 18′, 18″ will haverespective barrier layers B₁, B₂ that are equivalent to each other, butare smaller than the barrier layer B of the pore 18 due to theproportionality to the anodization voltage at the final stage (1.3 nmper volt). When forming secondary pores 18′, 18″, it is to be understoodthat the electrolyte may be changed from the electrolyte used to formthe pore 18. When secondary pores 18′, 18″ are formed in the template16′, it is to be understood that the process will continue with theanisotropic etching, continued aluminum anodization, tantalumanodization, and template 16′ removal as described further hereinbelow.These processes will form tracks 31 and paths 32 (described furtherhereinbelow) beneath each of the secondary pores 18, 18″. It is to befurther understood that when secondary pores 18′, 18″ are formed in thetemplate 16′, the tracks 31 and paths 32 will be formed in the barrierlayers B₁, B₂ such that super nano-pillars 20 are formed adjacent (inone embodiment, below) each secondary pore 18′, 18″.

Referring now to, and as illustrated in, FIG. 1C, embodiments of themethod further include partially etching the template 16′. Partialetching may be accomplished via anisotropic etching, which furtherdefines the pores 18 and modifies the template 16′. Anisotropic etchingallows control over the size of the pores 18. This etching processfurther defines the already formed pores 18, and in many instancesincreases the diameter of the formed pores 18. This etching process alsodissolves some of the alumina at the bottom of each pore 18 to formtracks 31 therein (see FIG. 1C-A for an enlarged view of the tracks 31).A track is a weakened portion of the alumina 16′, where the alumina 16′starts to dissolve. It is believed that tracks 31 form when protonsdiffuse into the alumina, which initiates alumina dissolution. As such,the tracks 31 may be more predominate at the surfaces directly exposedto the etching process. Additionally, it is believed that the track 31that is formed may be different depending upon the electrolyte usedduring the first anodization step. For example, an H₂CrO₄ electrolyteforms an oxide that has a relatively small concentration of incorporatedelectrolyte ions. This may affect the configuration of the tracks 31when protons are diffused into the alumina 16′ during etching. It is tobe understood that while not shown in FIG. 2C, tracks 31 may form inother portions of the template 16′ as well.

Anisotropic etching may be performed using diluted phosphoric acid (5vol. %) or a solution of sulfuric acid (20 vol. %). It is also believedthat a diluted form of a hydroxide such as, e.g., NaOH or KOH may beused for etching. The time for etching may vary, depending, at least inpart, upon the desirable average diameter for the final pores 18, thedesired height of the super nano-pillars 20 to be formed, the etchantused and its concentration, and/or the etching temperature. In anembodiment, the anisotropic etching time ranges from about 15 minutes toabout 45 minutes in instances where anisotropic etching is performedusing a diluted phosphoric acid (5 vol. %) at about 30° C. In thisembodiment, the etching time may be up to, in one example, about 30minutes, or may be less than 15 minutes (e.g., 5 minutes). The durationof anisotropic etching will contribute to how large an area will becovered by a set 24, 24′ of super nano-pillars 20, to how large the gapbetween adjacent sets 24, 24′ of super nano-pillars 20 will be, thefilling factor (pillar density), and in some instances, the height ofthe super nano-pillars 20.

The temperature for etching may also depend upon the process and etchantused. In one embodiment, the etchant temperature ranges from about 0° C.to about 100° C. depending, at least in part, on the type of etchantused. In an example, the etchant temperature ranges from about 20° C. toabout 40° C., for example, when a diluted phosphoric acid etchant isused.

As shown in FIGS. 1D and 1D-A, a second aluminum layer anodization isperformed to complete the anodization of the remaining aluminum layer 16(i.e., the aluminum that was not anodized to form the template 16′ shownin FIG. 1B). This second aluminum layer anodization step oxidizes theremaining aluminum layer 16 through to the tantalum layer 14, and thusintroduces another barrier layer B′ to the template 16′. When the lastremaining portions of the aluminum layer 16 are anodized, these portionsbecome alumina. As the anodization process continues, all of thealuminum is consumed and thus complete aluminum anodization is achieved.This second aluminum layer anodization step also forms paths 32 in thetemplate 16′ where the tracks 31 were formed during etching. During thisanodization process, it is believed that field assisted dissolving opensup the tracks 31 to form the paths 32.

In one embodiment not shown in the drawings, the aluminum anodizationprocess of FIG. 1D may be continued in the electrolyte (e.g., oxalicacid) that etches away the portion of the barrier layer B′ making up thebottom of the paths 32 by field assisted dissolving. Once the paths 32are opened through to the underlying tantalum layer 14, the electrolytemay be switched to one that will form a dense oxide from the layer 14.These electrolytes may be selected from citric acid (C₆H₈O₇), boric acid(H₃BO₃), ammonium pentaborate ((NH₄)₂B₁₀O₁₆ x 8H₂O), ammonium tartrate(H₄NO₂CCH(OH)CH(OH)CO₂NH₄), mixtures thereof, or another suitableelectrolyte. This electrolyte will be used in a tantalum anodizationprocess that forms a dense tantalum pentoxide layer 14′ that willultimately grow to form the nano-pillars 20 through the open paths 32.

Referring back to the embodiment shown in FIGS. 1D and 1E, eitheranodization is continued or another anodization current and voltage isapplied to initiate a plurality of steps that lead to the formation oftantalum pentoxide nano-pillars 20. In one embodiment, the initial andcontinued anodization of layer 14 is accomplished by employing thetantalum layer 14 as the anode of an electrolytic cell and employingplatinum, stainless steel, or any other appropriate material as thecathode, and applying a suitable anodization voltage and/or currentdensity to initiate the various processes described herein.

It is to be understood that in the embodiment shown in the figures, whenthe layer 14 starts to oxidize, the anodic alumina barrier B′ is stillpresent. This alumina barrier layer B′ is a dielectric layer between theelectrolyte and the metal (in this case, tantalum). During initialtantalum layer 14 anodization, at least the portion of the barrier layerB′ making up the bottom of the paths 32 is etched away by field assisteddissolving. This dissolution occurs when the electrolyte used is thesame kind of electrolyte used for porous anodization of layer 16. Forexample, dissolution will occur when oxalic acid (i.e., H₂C₂O₄) (or anyof the other electrolytes used to form porous alumina) is used as theelectrolyte. Dissolution opens up the paths 32 to enable growth oftantalum pentoxide super nano-pillars 20 therein. This is shown in FIGS.1E and 1E-A. It is believed that field assisted etching is taking placeon the bottom of any pore 18, where the distance between the layer 14and the electrolyte is the shortest. This etching process is fasterthan, e.g., chemical etching and thus readily opens up the paths 32.

After the paths 32 are formed, they may be subjected to further etchingin order to increase the diameter. This additional etching process maybe any chemical etching process.

As the anodization of the oxidizable material layer 14 (in this examplethe tantalum layer 14) continues, the oxidized form of the tantalum(i.e., tantalum pentoxide structure 14′) grows through the individualpaths 32 defined in the pores 18 of the template 16′ to form a pluralityof super nano-pillars 20. It is to be understood that the volume of thetantalum pentoxide that grows during the anodization of the tantalumlayer 14 should exceed the volume of the tantalum from which the oxideis formed so that the oxide squeezes into the paths 32. Tantalumanodization is continued at least until the nano-pillars 20 are formedin the paths 32. One super nano-pillar 20 is formed in each path 32, asshown in FIGS. 1E-A and 1E-A.

In one embodiment, the tantalum layer 14 is anodized at an appropriateanodization voltage and/or current density for an amount of timesufficient for the tantalum pentoxide super nano-pillars 20 to grow,inside their respective paths 32 defined in the pores 18, up to apredetermined height h (see FIG. 1E-A). In an example, the tantalumpentoxide super nano-pillars 20 grow until each super nano-pillar 20 hassubstantially the same predefined height h that terminates at an end 21(as shown in FIG. 1E-A). In the embodiment shown in FIG. 1E-A, theheight h of the nano-pillars 20 is equivalent to (or in otherembodiments shorter than) a height of the paths 32 within which thesuper nano-pillars 20 are grown. In one embodiment, as soon as the supernano-pillars 20 are grown to the predetermined height h, anodization isstopped.

The orientation of the super nano-pillars 20 is generally controlled bythe orientation of the paths 32. In the embodiments of the methoddepicted in the FIG. 1 series, the super nano-pillars 20 are oriented ina position that is substantially normal to the substrate 12. When thesuper nano-pillars 20 grow during the oxidizable material anodization,the geometry and/or dimensions of the super nano-pillars 20 will conformto that of the paths 32 within which the super nano-pillars 20 aregrowing. The dimensions of the template 16′ (and the paths 32) may becontrolled by the nature of the electrolyte selected, the electrolyteconcentration, the temperature of aluminum anodization, the currentdensity, the anodization voltage, duration of the etching process thatis performed, and/or the completeness of the anodization process.

Tantalum anodization also forms the dense oxide structure 14′ beneaththe template 16′. As anodization continues, both the interface betweenthe oxidizable material layer 14 and the formed anodic oxide (i.e.,oxide structure) 14′ and the interface between the anodic oxide 14′ andthe electrolyte (not shown) are planarized. The affect of planarizationat the interfaces is shown in FIG. 1E.

The oxidized form of the tantalum formed during the anodization of thetantalum layer 14 is a substantially pure oxide. As used herein, a“substantially pure oxide” refers to an oxide that may include someimpurities. Typically, dense oxides (such as the structure 14′) have asmaller amount of impurities as compared to porous oxides (such as thetemplate 16′). In one embodiment, the dense oxide includes a smallportion of the alumina (or other material forming the template 16′)and/or of the electrolyte. In one embodiment, the porous aluminatemplate 16′ may have up to about 15 wt % or up to about 18 wt % ofelectrolyte ions incorporated and/or absorbed/adsorbed therein.

After the super nano-pillars 20 are grown to the desirable height h, twodifferent processes may occur. In one embodiment, anodization stops, andthe entire structure (i.e., the multi-layered stack) is removed from theelectrolytic cell and the alumina template 16′ is removed (see FIG. 1F).In another embodiment, anodization is continued (see FIG. 1G).

Referring now to FIG. 1F, after the super nano-pillars 20 are grown, thetemplate 16′ is removed to form the nano-structure 100. In an example,the template 16′ is removed using a selective etching process that willremove the anodic alumina template 16′ without deleteriously affectingthe other features (e.g., 14′, 20). Selective etching may beaccomplished using an etchant solution (such as, e.g., H₃PO₄—CrO₃—H₂O)solution) at a temperature ranging from about 80° C. to about 95° C. Itis to be understood that etching may also be accomplished at atemperature outside of the foregoing range, but the duration of theetching may be affected. For instance, at a temperature lower than 80°C., the duration of the etching may be longer. In some cases, etchingmay also be accomplished at temperatures as high as the boiling point ofthe solution (such as, e.g., about 100° C.). In this embodiment, H₃PO₄etches the alumina and the CrO₃ passivates aluminum etching (this isparticularly desirable when working with patterned aluminum andlocalized alumina). In one example, the etchant solution includes about92 g of H₃PO₄, about 32 g of CrO₃, and about 200 g of H₂O, although itis to be understood that the components of the etchant may vary. It hasbeen found that the nano-pillars 20 can withstand this particularetching process for more than one hour, while the anodic aluminatemplate 16′ is etched away at a rate of about 1 micron per minute.Other etching solutions that may be used include hydroxide solutionssuch as, e.g., NaOH, KOH, etc. The alumina template 16′ may also beetched using a 5% H₃PO₄ solution at 30° C., H₂SO₄, etc. Etching may beaccomplished, if desired and/or required, in a lateral direction to adistance of about 100 μm, and in some instances even further.

The nano-structure 100 formed via the method of FIGS. 1A through 1F isshown in FIG. 1F. This embodiment of the nano-structure 100 includesthree sets 24 of free standing super nano-pillars 20 where each set 24is positioned a spaced distance from each other set 24. The spaceddistance is dictated by the template 16′, which is now removed. In oneembodiment, the free-standing super nano-pillars 20 can bend and absorbenergy of acoustic waves. In another embodiment, the free-standingnano-pillars 20 may be used, due in part to the high surface area, as asubstrate for the deposition of catalytically active ingredients and forsensing.

Referring now to FIGS. 1G and 1G-A, the tantalum layer 14 anodizationcontinues at an appropriate anodization voltage and/or current densityfor an amount of time sufficient for the tantalum pentoxide supernano-pillars 20 to grow through the entire thickness or height of theirrespective paths 32. When the tantalum pentoxide super nano-pillars 20reach the top of the portion of the template 16′ defining the paths 32,the tantalum pentoxide continues to grow over these portions of thetemplate 16′. More specifically, upon reaching the top of the paths 32,the tantalum pentoxide of one super nano-pillar 20 spreads over the topof the template portions 16′ and merges with the tantalum pentoxide ofan adjacent super nano-pillar 20. The spreading of the tantalumpentoxide from the super nano-pillars 20 occurs in all directions sothat a dense cap layer 22 can be formed over the pillars 20 and over theportions of the template 16′ defining the paths 32. In other words, therespective ends 21 (see FIG. 1E-A) of the super nano-pillars 20 continueto grow, and ultimately spread across the surface of the portions of thetemplate 16′ and merge together to form a substantially continuous caplayer 22 (i.e., a cap layer 22 that is absent of, or includes a verysmall number of holes, gaps, or the like).

The cap layer 22 has a thickness that is controllable by the anodizationof the tantalum layer 14. In one example, the thickness increases as theanodization voltage increases. In many cases, the total mass of thenano-structure 100′ (see FIG. 1H) may be controlled by adjusting thethickness of the cap layer 22 or a lateral area of the cap layer 22. Inan example, the cap layer 22 should be thick enough so that the caplayer 22 is dense. It is to be understood that the cap layer 22 may beformed whenever there is enough of layer 14 and the voltage is highenough to grow the oxide 14′ through and out of the paths 32. In oneembodiment, formation of the cap layer 22 consumes any remaining layer14.

After the cap layer 22 has been formed, the template 16′ may be removedusing any of the embodiments of the template 16′ removal processdescribed above. The resultant nano-structure 100′ is shown in FIG. 1H,where each set 24′ of super nano-pillars 20 has a cap layer 22 thereon.

It is to be understood that, in some cases, it may be desirable tomodify the surface chemistry of the super nano-pillars 20 and/or the caplayer 22, for example, to improve the chemical robustness of thenano-structure 100, 100′, to tune the contact angle of these surfaces inorder to improve wettability or to stop wetting, to change the acidityof zeta potential of these surfaces so that the surfaces may have adifferent affinity to different chemicals, etc. Modification of thesurface chemistry may be accomplished, for example, by depositing amaterial on a surface of the super nano-pillars 20 and/or the cap layer22. Deposition of the material may be accomplished, for example, byatomic layer deposition, chemical vapor deposition, metal organicchemical vapor deposition (MOCVD), electrochemical deposition, and/orthe like. In an example, the material may be conformally deposited overthe entire surface of the selected nano-pillars 20 and/or the cap layer22 at a thickness ranging from about 4 nm to about 8 nm. In anotherexample, the thickness of the deposited layer is about 6 nm. Someexamples of the materials that may be deposited on the nano-pillars 20and/or the cap layer 22 include aluminum oxide, zirconium oxide,titanium oxide, silicon dioxide, tungsten oxide, zinc oxide, hafniumoxide, or combinations thereof.

To further illustrate embodiment(s) of the present disclosure, examplesare given herein. It is to be understood that these examples areprovided for illustrative purposes and are not to be construed aslimiting the scope of the disclosed embodiment(s).

EXAMPLES Example 1

A nano-structure was formed using a multi-layer structure of aluminum(300 nm) on tantalum (50 nm). The aluminum was anodized in 2% oxalicacid at 30 V for about 3 minutes and 45 second (i.e., about 90 secondsbefore the complete aluminum anodization, which was estimated accordingto option 3 in FIG. 2) to form a porous anodic alumina template having athickness of about 300 nm. The template was anisotropically etched toincrease the pore diameter and to form tracks inside each pore. Etchingwas accomplished using a 5% H₃PO₄ solution for about 30 minutes at atemperature of about 30° C. Thereafter, the remaining aluminum wasfurther anodized. This second aluminum anodization was accomplishedusing 2% oxalic acid at 30 V until the current density decreased andreached a steady state. This formed paths where tracks had been formed.Tantalum anodization was performed in the same oxalic acid electrolyteusing a current density of about 0.8 mA/cm² until the voltage reachedabout 120 V, and then anodization was continued for 2 minutes at 120 V.In this example, the entire tantalum layer was anodized. Anodization ofthe tantalum formed a tantalum pentoxide layer having a thickness (whichincludes the oxide layer 14′ and the super nano-pillars 20) of about 140nm. It was noted that the overall tantalum pentoxide thickness,including the height of the super nano-pillars, was limited (in part) bythe available thickness of the tantalum layer. It is believed that theheight of the super nano-pillars is also limited by the height of thepaths. The alumina template was removed by etching the template inH₃PO₄+CrO₃+H₂O solution as described herein. FIG. 5A illustrates atransmission electron micrograph (TEM, scale bar is 20 nm) cross-sectionof a portion of the nano-structure including sets of super nano-pillars,and FIG. 5B illustrates a scanning electron micrograph (SEM, scale baris 200 nm) image of a perspective view of the same nano-structure.

Example 2

A nano-structure was formed using a multi-layer structure of aluminum300 nm) on tantalum (50 nm). The aluminum was anodized in 2% oxalic acidat 30 V for about 3 minutes and 45 second (i.e., about 90 seconds beforethe complete aluminum anodization, which was estimated according tooption 3 in FIG. 2) to form a porous anodic alumina template having athickness of about 300 nm. The template was anisotropically etched toincrease the pore diameter and to form tracks inside each pore. Etchingwas accomplished using a 5% H₃PO₄ solution for about 15 minutes at atemperature of about 30° C. to form the tracks. Thereafter, the aluminumwas further anodized using 2% oxalic acid at 30 V until the currentdensity decreased and reached a steady state. This formed paths wheretracks had been formed. Tantalum was then anodized in the same oxalicacid electrolyte using a current density of about 0.8 mA/cm² until thevoltage reached about 120 V, and then anodization was continued for 2minutes at 120 V. Anodization of the tantalum formed a tantalumpentoxide layer having a thickness (which includes the oxide layer 14′,the super nano-pillars 20, and the cap 22) of about 165 nm. It was againnoted that the overall tantalum pentoxide thickness, including theheight of the super nano-pillars, was limited by the available thicknessof the tantalum layer. The alumina template was removed by etching thetemplate in H₃PO₄+CrO₃+H₂O solution as described herein. FIG. 6Aillustrates transmission electron micrograph (TEM, scale bar of 20 nm)cross-sections of a portion of the nano-structure including sets ofsuper nano-pillars having a cap layer deposited on each set, and FIG. 6Billustrates a scanning electron micrograph (SEM, scale bar of 200 nm)image of a perspective view of the same nano-structure.

In this example, the short etching time (when compared to Example 1)contributed to the creation of the cap layer. In this example, whencompared to Example 1, the paths were smaller and the tantalum pentoxidefilling factor was smaller, and thus the amount of tantalum was enoughto fill the paths and allow the tantalum pentoxide to grow out of thepaths and over portions of the template.

While several embodiments have been described in detail, it will beapparent to those skilled in the art that the disclosed embodiments maybe modified. Therefore, the foregoing description is to be considerednon-limiting.

What is claimed is:
 1. A method for making a nano-structure, comprising:partially anodizing an aluminum layer in an electrolyte and at a voltageto form a porous anodic alumina structure including a plurality ofpores, wherein each of the plurality of pores has a bottom portion thatis defined by an alumina barrier layer of the porous anodic aluminastructure, the aluminum layer being positioned on an oxidizable materiallayer; partially anisotropic etching the porous anodic alumina structureto form tracks in the alumina barrier layer at the bottom portion ofeach of the plurality of pores within the porous anodic aluminastructure; further anodizing a remaining portion of the aluminum layerin the electrolyte and at the voltage until a current density decreasesand reaches a steady state, thereby completing anodization of thealuminum layer and forming open paths where the tracks had been formed;and anodizing the oxidizable material layer to form an oxide, wherebythe oxide grows through the open paths formed within the porous anodicalumina structure to form a set of super nano-pillars.
 2. The method asdefined in claim 1 wherein during the anodizing of the oxidizablematerial layer, the oxide grows through the open paths to form multiplediscrete sets of super nano-pillars.
 3. The method as defined in claim1, further comprising forming a cap layer on the set of supernano-pillars during the anodizing of the oxidizable material layer. 4.The method as defined in claim 3 wherein prior to forming the cap layer,the method further comprises: controlling a thickness of the oxidizablematerial layer; and controlling a voltage during anodizing of theoxidizable material layer.
 5. The method as defined in claim 1, furthercomprising removing the porous anodic alumina structure.
 6. The methodas defined in claim 1, further comprising: estimating a duration forperforming complete anodization of the aluminum layer; and determining aduration for the partial anodization of the aluminum layer based uponthe estimated duration for performing the complete anodization of thealuminum layer.
 7. The method as defined in claim 1 wherein thepartially anisotropic etching of the porous anodic alumina structure isaccomplished using about 5 vol. % H₃PO₄ at about 30° C. for a timeranging from about 5 minutes to about 45 minutes.
 8. The method asdefined in claim 1 wherein the electrolyte is chosen from H₂SO₄, H₃,PO₄, H₂C₂O₄, H₂CrO₄, and mixtures thereof.
 9. The method as defined inclaim 1 wherein the anodizing of the oxidizable material layer isaccomplished using a second electrolyte selected from the groupconsisting of H₂SO₄, H₃, PO₄, H₂C₂O₄, H₂CrO₄, and mixtures thereof. 10.The method as defined in claim 1 wherein the tracks are weakenedportions in the alumina barrier layer.
 11. The method as defined inclaim 1 wherein the voltage is 30 V, and the partial anodization isaccomplished for about 3 minutes and 45 seconds.